Level probes for shaft furnaces are well known and are used for determining the level of the charging surface of a furnace, especially for determining the beginning and end of a charging session. These probes must satisfy a plurality of requirements and criteria of which the effects are often contradictory. For example, as regards the operating speed of the probe foot for lowering it onto the charging surface and for raising it, it is desirable that the speed of the drum be relatively high in order to avoid losses of time. In contrast, if the lowering speed of the probe foot is high, the latter risks penetrating into the charging material and thus falsifying the measurement results. The speed of the motor must therefore be determined so as to find a reasonable compromise between these two constraints.
Another problem is the increasing in the torque exerted on the motor during the lowering of the probe foot onto the charging surface. In fact, as the unwinding progresses, the weight of the chain or cable is added to that of the probe foot, and this increase in torque, coupled with the same braking torque of the motor, increases the lowering speed of the probe foot. Now it is exactly the opposite which is desired, that is to say a slowing of the probe foot as it approaches the charging surface, precisely to prevent it from penetrating into the charging surface.
Another problem is that the necessary measures have to be taken to prevent the probe foot from overturning after it comes into contact with the charging surface, that is to say it is necessary to ensure that the probe foot remains upright on the charging surface and descends together with this, without overturning, otherwise the measurement results would be falsified. Consequently, when the probe foot touches the charging surface, the torque of the motor must be increased, care nevertheless being taken to ensure that this increase in torque is not too great, to avoid lifting the weight again.
Slip-ring motors with rotor resistors which can be connected and disconnected, as required, in order to modify the torque of the motor have conventionally been used to satisfy the above discussed constraints.
When the probe is to be lowered onto the charging surface, the motor torque is adjusted by means of the rotor resistors to a specific value below the lowering torque generated by the probe foot and corresponding to a specific lowering speed. To compensate the increase of the lowering torque as a result of the increase in weight of the unwinding cable and to keep the lowering speed of the probe foot constant, the torque of the motor is increased progressively by means of the rotor resistors.
At the moment when the probe foot touches the charging surface, the value of the rotor resistors is further reduced, in order to increase the motor torque to a sufficient value to keep the probe foot in an upright position, but without lifting it from the charging surface, so as to allow it to descent together with the latter.
The various modifications of the rotor resistors to determine the torque of the motor are carried out automatically under the control of means for detecting the position of the probe foot, this being by means of a predetermined calibration which makes it possible to adjust the various moments of modification of the resistors according to the position of the probe foot.
Although this system has been in operation for years, it nevertheless has a certain number of disadvantages which become increasingly evident as technological advances are made. Thus, for example, the calibration of the probe and the adjustment of the various values of the rotor resistors are jobs for specialists and have to be repeated regularly because the running conditions of the furnace bring about unforeseeable changes in the lowering torque of the probe. These changes in the torque can be caused by fluctuations in the temperature of the lubricant, the soiling of the drum and of the chain, the wear of the glands of the bearings of the motor and of the reducer, etc. Moreover, the calibration and adjustment of the values of the resistors can be carried out only during a shutdown of the furnace. Now it goes without saying that a poor adjustment of the values of the resistors gives rise to measuring inaccuracies.
To have a sufficiently wide range of adjustment and thereby make this work easier, while at the same time allowing for the variations in the mechanical efficiency, it is necessary to provide somewhat heavy equipment, particularly a probe foot weighing between 100 and 250 kilos. Because of this high weight, it is necessary to operate at relatively low speeds, to prevent the probe foot from penetrating into the charging material.
Furthermore, the slip-ring motor requires regular maintenance because of the wear of its brushes and rotor rings